As a three-dimensional ultrasonic picture generation technique in ultrasonic diagnostic equipment, a ray-casting method is well known. This method is widely used to draw a three-dimensional picture of a fetus, in a medical field (especially, an obstetrics and genecology department). Here, when in a case of drawing the three-dimensional picture of the fetus, the ray-casting method is simply used, the placenta and the uterine wall, which cover the fetus targeted for the three-dimensional picture generation although they are not targeted for the three-dimensional picture generation, are drawn which disables the three-dimensional picture of the desired fetus to be drawn. Thus, the conventional method was required to carry out a clipping operation in which a doctor or a sonographer manually removes the placenta and the uterine wall that shielded the fetus. This clipping operation was the technique whose automation was desired for a long time because the much labor and the long time were required for the doctor or sonographer.
So, an automatic obstacle removing method indicated by the following patent document 1 is proposed. FIG. 7 shows a block diagram of three-dimensional ultrasonic picture diagnostic equipment that contains a conventional automatic obstacle remover described in the patent document 1. In FIG. 7, a transmitting/receiving unit 701 transmits an ultrasonic beam through a two-dimensional array probe 700 to an object and receives a reflection wave from the object. A detecting circuit 702 detects a received signal from the transmitting/receiving unit 701. A three-dimensional picture processor 703 generates a volume data from the received signal from the detecting circuit 702 and carries out a ray-casting calculation and then generates a three-dimensional picture. A display 704 displays the three-dimensional picture generated by the three-dimensional picture processor 703. A movement averaging unit 705 smoothes the received signal processed by the detecting circuit 702. A comparator 706 compares the signal smoothed by the movement averaging unit 705 and a predetermined threshold TH to judge an amniotic fluid and then judges the drop portion of a received signal level. A ray-casting start position setter 707 determines a ray-casting start position in accordance with the drop portion (amniotic fluid portion) of the received signal level judged by the comparator 706 and outputs to the three-dimensional picture processor 703.
Here, the portion related to the automatic obstacle removal as the conventional technique is the portion surrounded by a dotted line in FIG. 7. This portion will be described below in detail. The comparator 706 can select two algorithms (A), (B) in order to detect the drop portion (amniotic fluid portion) in the depth direction of the received signal level. The algorithm (A) is the algorithm for detecting the drop portion in the depth direction of the received signal level (brightness value) in its original state, as shown in FIG. 8. The comparator 706 sequentially scans the received signal in the depth direction and stores a received signal depth D1 when the received signal becomes lower than the threshold TH=E1(<E2). Next, the comparator 706, while further advancing the scanning in the depth direction, stores a received signal depth D2 at this time when the received signal becomes higher than the threshold TH=E2. When the scanning of the received signal has been ended, the received signal depths D1, D2 are outputted to the ray-casting start position setter 707.
The algorithm (B) is the algorithm, which as shown in FIG. 9, integrates the received signal levels (brightness values) in the depth direction shown in FIG. 8, and determines a cumulative brightness value, and in accordance with the gradient (differential value) of the cumulative brightness values, indirectly detects the drop portion (amniotic fluid portion) in the depth direction of the received signal level. The comparator 706, before carrying out the comparing process, carries out the cumulative addition in the depth direction of the received signals. Next, the comparator 706 compares the previously-generated gradient in the cumulative addition with the thresholds TH=F1, F2 (F1<F2) and consequently determines the received signal depths D1, D2, similarly to the algorithm (A). At first, the comparator 706 sequentially scans the gradient of the cumulative addition value of the received signal in the depth direction and stores the received signal depth D1 when the gradient of the cumulative addition value of the received signal becomes smaller than the threshold TH=F1. Next, the comparator 706, while further advancing the scanning in the depth direction, stores the received signal depth D2 at this time when the gradient of the cumulative addition value of the received signal becomes greater than the threshold F2. When the scanning of the cumulative addition value of the received signal has been ended, the received signal depths D1, D2 are outputted to the ray-casting start position setter 707.
In the ray-casting start position setter 707, a middle depth=(D1+D2)/2 of the received signal depths D1, D2 determined by the comparator 706 is defined as a ray-casting start depth A, and outputted to the three-dimensional picture processor 703. For this reason, the three-dimensional picture processor 703 does not generate the three-dimensional pictures of the obstacles and a part of the amniotic fluid, which are located at the positions shallower than the ray-casting start depth A, and then generates the three-dimensional pictures of a part of the amniotic fluid and the fetus, which are located at the positions deeper than the ray-casting start depth A. As mentioned above, the conventional method, since containing the comparator 706 and the ray-casting start position setter 707, judges the drop portion of the received signal that serves as the amniotic fluid portion, and automatically removes the placenta, the uterine wall and the like (the obstacles) which cover the fetus, and consequently draws only the fetus.    Patent Document 1: Japanese Patent Application Publication (JP-P 2001-145631) (FIG. 4 and FIG. 6)